It is well-known that sunlight radiations ranging from 290 to 400 nm are noxious to the organic materials, among which human skin too, and particularly those radiations with wavelength between 290 and 320 nm, the so-called UV-B radiations, are responsible of the occurrence of erythema and sunburns, whose severity depend on exposition length.
It has been ascertained that also radiations comprised between 320 and 400 nm, so-called UV-A, which are responsible of skin tanning, can cause alterations and important damages in skin especially in the case of sensitive skin or in case of continuous exposure to the radiation. It has been shown that UV-A radiation, beside causing damages to elastin and collagen, the consequence of which is skin ageing, can also be the cause of a number of phototoxic and photoallergic reactions. Moreover, the noxious action of UV-B can also be enhanced by the presence of UV-A (Willis et al. , Journal of Investigative Dermatology, vol. 59, 416, 1072).
In order to provide a protection against noxious UV-B radiation, a number of compounds are well-known and also used in cosmetic compositions, such as for example cinnamic acid, 4-aminobenzoic acid, benzylidenecamphor and benzophenone derivatives.
On the contrary, to date no sufficiently suitable products are available for the protection against UV-A, notwithstanding the patent literature provides many compounds as UV absorbers, but the practical result of them has not been satisfying.
A commercially available product is 2-hydroxy-4-methoxybenzophenone, whose maximum absorption at about 325 nm is too low to give an to effective protection, moreover its solubility in the solvents, usually used in cosmetics, is very low, thus making its use difficult. Another compound which is now used in practice is a dibenzoylmethane derivative, which however is insufficiently photostable ( Int. J. Cosm. Science 10, 53 1988 ) . Therefore, the sun protecting compositions containing these compounds can not guarantee a sufficient protection against UV-A, since the UV absorbers therein used are whether too weak (such as benzophenone derivative) or are too fast deteriorated by the radiation itself (such as dibenzoylmethane derivative).
Accordingly, nowadays cosmetic industry has not suitable products available for the effective protection of skin from sun radiations comprised between 320 and 400 nm.